This invention relates to screw rotors of screw compressors, and, more particularly, to screw rotors suitable for use with a dry type screw compressor in which the rotors are made to rotate while meshing with each other by synchronizing means, without the rotors coming into contact with each other.
Generally, in dry type screw compressors suitable for use in applications where mingling of oil in the gas discharged from a screw compressor is not desirable, transmission of rotation between a pair of screw rotors is effected through synchronizing means mounted at shaft portions outside the working chambers of the rotors, and at this time the rotors rotate while meshing with each other without coming into contact with each other. The screw rotors of this type of screw compressors have their teeth heated to a higher temperature during operation than the screw rotors of an oil-cooled type screw compressor in which oil is injected into the working chambers for the rotors to mesh with each other in so as to lubricate, cool and seal the two rotors, so that the teeth are subjected to thermal deformation during operation and their shape greatly differs during operation from their shape in inoperative condition in which the temperature is normal. Thus, when the two rotors are designed, it is necessary to select a dimensional relation for them in such a manner that the rotors are prevented from coming into contact with each other and with the casing while a minimum clearance is kept therebetween during operation.
In rotor design of the prior art, however, it has been the usual practice to roughly decide upon the clearance between the two rotors and between the rotors and the casing, so that the clearances provided between the rotors and between the rotors and the casing have no theoretical basis. This has caused problems to arise with regard to the efficiency of the screw compressors.
More specifically, in one practical process known in the art for providing a clearance between the two rotors of a screw rotor, a basic tooth form, for example, given to the male rotor, and a predetermined clearance is provided in the direction of the normal to the tooth form of the female rotor by taking into consideration deformation of the rotors that would occur due to thermal expansion during operation.
This process for deciding the clearance between the rotors is not considered the best because the clearance given to the rotors by this process does not have an optimum value selected by studying in detail the thermal expansion of the rotors and the clearance between the rotors as measured during operation, since the tooth form will undergo deformation in different manners due to thermal expansion and the deformation may vary depending on the tooth form of the rotors.
In, for example, U.S. Pat. No. 3,414,189, for providing a clearance between the rotors, wherein a smaller clearance is given to the rotors in a region in which relative sliding movement between the teeth of the rotors meshing with each other is small and a larger clearance is given to the teeth of the rotor in other regions.
However, this process would not be considered to quantitatively take into consideration the thermal deformation to which the two rotors of the screw compressor would be subjected during operation.